CELT

Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.wikipedia
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Opus (audio format)

OpusOpus audio format.opus
Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).
Opus combines the speech-oriented linear predictive coding SILK algorithm and the lower-latency, MDCT-based CELT algorithm, switching between or combining them as needed for maximal efficiency.

Ogg

.oggOGMogg123
Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).
Being a container format, Ogg can embed audio and video in various formats (such as Dirac, MNG, CELT, MPEG-4, MP3 and others) but Ogg was intended to be, and usually is, used with the following Xiph.org free codecs:

SILK

Skype SILKSteel Industrials Kerala Limited
Development is going on only for its hybridised form as a layer of Opus, integrated with SILK.
SILK is a foundation (with CELT) of the hybrid codec Opus (at the time called "Harmony") that was submitted to the IETF in September 2010, and was chosen as the final candidate for the new standard.

Mumble (software)

MumbleMumble room
Despite the format not being finally frozen, it was being used in many VoIP applications such as Ekiga and FreeSWITCH, which switched to CELT upon entering soft-freeze in January 2009, as well as Mumble, TeamSpeak and other software.
Mumble uses the low-latency audio codec Opus as of version 1.2.4, the codec that succeeds the previous defaults Speex and CELT.

Xiph.Org Foundation

Xiph.orgXiphThe Xiph.Org Foundation
Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).
CELT – an ultra-low delay lossy audio compression format that has been merged into Opus, and is now obsolete

Code-excited linear prediction

CELPcode excited linear predictionCode-excited linear prediction (CELP)
It borrows ideas from the CELP algorithm, but avoids some of its limitations by operating in the frequency domain exclusively.
CELT is a related audio codec that borrows some ideas from CELP.

Ekiga

Despite the format not being finally frozen, it was being used in many VoIP applications such as Ekiga and FreeSWITCH, which switched to CELT upon entering soft-freeze in January 2009, as well as Mumble, TeamSpeak and other software.
Audio codec algorithms: iLBC, GSM 06.10, MS-GSM, G.711 A-law, G.711 µ-law, G.726, G.721, Speex, G.722, CELT (also G.723.1, G.728, G.729, GSM 06.10, GSM-AMR, G.722.2 [GSM‑AMR-WB] using Intel IPP)

FreeSWITCH

FreeSWITCH IPBX
Despite the format not being finally frozen, it was being used in many VoIP applications such as Ekiga and FreeSWITCH, which switched to CELT upon entering soft-freeze in January 2009, as well as Mumble, TeamSpeak and other software.
CELT and Opus

Royalty-free

royalty free
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Lossy compression

lossylossy data compressioncompressed
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Data compression

compressionvideo compressioncompressed
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Free software

freefree-softwarefreely
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Audio codec

audio codecsaudio codingcodec
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Latency (engineering)

latencylow latencylow-latency
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Software patent

software patentspatentpatents
The algorithms are openly documented and may be used free of software patent restrictions.

Internet Engineering Task Force

IETFInternet Engineering Task Force (IETF)working group
Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).

Vorbis

Ogg VorbisOGGaoTuV
CELT was meant to bridge the gap between Vorbis and Speex for applications where both high quality audio and low delay are desired. Like Vorbis, CELT is a fullband (entire human hearing range) general-purpose codec, i.e. not specialized for special types of audio signals and therefore different from its sibling project Speex.

Speex

SPX.spxlibspeex
CELT was meant to bridge the gap between Vorbis and Speex for applications where both high quality audio and low delay are desired. Like Vorbis, CELT is a fullband (entire human hearing range) general-purpose codec, i.e. not specialized for special types of audio signals and therefore different from its sibling project Speex.

Frequency domain

frequency-domainfrequencyspectral analysis
It borrows ideas from the CELP algorithm, but avoids some of its limitations by operating in the frequency domain exclusively.

Sampling (signal processing)

sampling ratesamplingsample rate
It can use a sampling rate from 32 kHz to 48 kHz and above and an adaptive bitrate from 24 kbit/s to 128 kbit/s per channel and above.

BSD licenses

BSDBSD license3-clause BSD license
There are no known intellectual property issues pertaining to the CELT algorithm, and its reference implementation is published under a permissive open-source license (the 2-clause BSD).

Hearing range

pitch perceptionaudible rangerange of human hearing
Like Vorbis, CELT is a fullband (entire human hearing range) general-purpose codec, i.e. not specialized for special types of audio signals and therefore different from its sibling project Speex.

Transparency (data compression)

transparenttransparencydata compression transparency
The format enables for transparent results at high bitrates, as well as very decent quality at lower bitrates.

MP3

.mp3MP3 downloadmp3s
All in all, the compression capabilities are said to be significantly superior to those of MP3, and as another useful feature for realtime applications like telephony, CELT's audio quality at lower bitrates are even on par with HE-AAC v1, thanks to the band folding.